Tag: sol

Those of you reading The Martian, our ACS ChemClub book selection, are aware of the special timekeeping system that is used for Mars. Instead of keeping track of time by days, it is measured in sols. The main reason for this difference is the way Mars travels through space.

Similar to Earth, Mars moves in an orbit around the sun while at the same time spinning around its own axis. There are a couple of big differences though. For one, it takes Mars about 687 days to make one trip around the sun, which constitutes one Martian year. This is compared to a year on Earth that takes approximately 365 days. The other big difference is how long it takes Mars to spin on its axis, which fictional astronaut Mark Watney points out in the book (p. 18) takes 39 minutes longer than an Earth day. This makes the Martian sol 2.7 percent longer. This might not sound like much difference, but consider that after just two weeks the change would be over 30%, or 7 hours time difference.

To avoid confusion, a day on Mars is called a sol, short for a solar day. You might wonder why this is necessary, and before our modern era of space exploration it didn’t matter as much. But as scientists built and sent remote spacecraft to orbit and land on Mars, it became very important. Spacecraft, especially those that landed on the surface, were dependent on sunlight to operate their solar cells and provide electricity. Also, it was important to know when it was light enough to take pictures or videos outside, along with many other tasks. It is also much colder during the Martian nighttime compared to the hours when the sun is up. If the scientists on Earth worked on local time, it wouldn’t correspond to time on Mars.

Some NASA scientists work and live on Martian time, an experience they found to be very disorienting and stressful. After just 18 days their noon comes at midnight, and soon they are having to sleep all day to get ready for their work sol. Perhaps it was easier for Mark Watney to adjust, considering all his clues for waking and sleeping were normal every sol. At any rate, worrying about how long his sol lasted was among the least of his concerns!

If you have any comments or questions about keeping time on Mars share your thoughts with us in the comment section and on Facebook or Twitter using #ACSChemClubBook.